The present invention relates to the procedures used for determining traces of uranium present in an aqueous solution by spectrofluorimetry.
When it is wished to determine uranium in trace state in solutions, e.g. in radioactive effluents, it is known that spectrofluorimetry is the most suitable and most widely used method (cf e.g. the article entitled "Determination of uranium traces in a reprocessing plant", AIEA Nuclear Safeguards Technology 1982, Vol. 1, 260/25).
It is also known that when working in a pure solution, the detection limit of such a method is close to 10 .mu.gl.sup.-1 and that this limit is a consequence of the presence of spurious emission phenomena, which are superimposed within the solution on the fluorescent spectrum of the uranium. FIG. 1 illustrates, as a function of the time plotted on the abscissa, the decrease or decay of the fluorescence intensities plotted on the ordinate of the actual uranium and all the spurious compounds in the solution. This superimposing during a time t.sub.1 (up to the total decay of the spurious emission) of the spurious fluorescence and the fluorescence of the uranium makes it difficult to interpret the results. In current cases, time t.sub.1 is approximately 20 .mu.s, whilst uranium decay requires approximately 400 .mu.s.
However, for certain applications, such as e.g. the determination of uranium in surface water or solutions containing large quantities of inhibitor elements, it is necessary to use a more sensitive procedure making it possible to measure traces with levels of approximately 1 .mu.gl.sup.-1.
Canadian Pat. No. 1,064,726 entitled "Apparatus and method for uranium determination" discloses a method making it possible to achieve such performance levels through the use of a pulsed laser as the excitation source. The corresponding equipment, marketed by the Canadian Company SCINTREX Limited under the name "UA3", is based on the fact that it is possible to see in FIG. 1 that the fluorescence lifetime of the uranium is longer than that of the organic substances leading to the spurious disturbances referred to hereinbefore.
Thus, by carrying out the fluorescence measurement at the end of an adequately long time after each shot to ensure that the emission of organic substances has completely disappeared, it is then possible to hope that a signal specific to the uranium will be measured.
Under certain conditions, this procedure makes it possible to lower the detection limit to a content of around 0.05 .mu.gl.sup.-1 for said equipment, which is adequate for the determination of assay of surface water, which is e.g. generally carried out during prospecting at geological sites.
However, a known apparatus still suffers from disadvantages, which are sometimes highly prejudicial and which can essentially be grouped around three main points indicated below.
(1) The optical device provided in the apparatus does not have a dispersive system making it possible to obtain the spectrum giving the fluorescence intensity as a function of the emission wavelength. However, as this spectrum is perfectly characteristic of uranium, its recording is the only way of ensuring that the actual measured signal corresponds to uranium.
(2) The apparatus has a time measuring window positioned in a fixed manner at 30 microseconds after each laser shot. This operating procedure is based on the hypothesis according to which the signal received after 30 microseconds can only come from uranium, i.e. that time t.sub.1 of FIG. 1 is always less than 30 microseconds, but this hypothesis is not always satisfied. Moreover, in certain cases, particularly when the uranium is mixed with fluorescence inhibitors, the signal due to the uranium decreases much faster and becomes virtually 0 at 30 seconds. In this case, it is indispensable to be able to move the window in order to optimize the signal.
(3) The Scintrex apparatus does not make it possible to measure the lifetime of the excited state of the uranium or, and this amounts to the same thing, to follow the decay of its fluorescence over a period of time. However, this constitutes vital information concerning the composition of the solution and it forms the basis for the present invention.
Thus, the determinations carried out with the Canadian apparatus according to the method of known additions and this consists of measuring the intensity of a given fluorescent line of the uranium,
(a) for the solution to be determined,
(b) for solutions derived from the preceding solution and in which known uranium quantities have been added on each occasion.
This known method suffers from the serious disadvantage of requiring the successive preparation of at least two or three solutions, preventing any fast measurement and of leading to the destruction of the initial solution to be determined.